The invention relates to a novel laboratory test for evaluating the tendency of a gasoline, which may contain additives, to produce deposits during running of an engine.
Deposits from fuels and lubricants form in an engine throughout its running. Such deposits are produced by heat in an oxidising medium in the various elements of the feed circuit (carburettor, injectors, inlet valves) and in the combustion chambers (cylinder head, piston crown, exhaust valve, spark plug). They cause running perturbations by degrading conduit tuning, increasing emissions or even destroying the engine.
Numerous additives and additive formulations have been developed to prevent such fouling,. They are generally polymeric surfactants comprising a lipophilic portion which ensures solubility and a polar portion which can be adsorbed on the engine walls to protect them or onto insoluble particles to keep them in suspension. In the case of spark ignition engines, they are also usually associated with a carrier oil which can be a polyolefin or a polyether. Such additives are effective in reducing and even preventing deposits from being formed in the feed circuit but do not reduce the deposits in the combustion chambers. It has also been shown that such additives can cause an increase in such deposits.
The tendency of a gasoline to produce deposits when an engine is running is generally evaluated by bench engine tests but such tests are long, expensive and require unwieldy equipment as well as a large quantity of fuel. Different engine tests have to be carried out to measure fouling in different parts of the engine. The engine test which is most frequently used in Europe to evaluate the ability of a gasoline or an additive-containing gasoline to keep the inlet valves clean is the Mercedes 102E engine test which lasts 60 hours, requiring about 300 liters of fuel.
A number of laboratory tests have been proposed in the past to simulate fouling in the different parts of an engine but until now none have led to sufficiently predictive results to be generally used. In 1997, an investigative group (IF031) charged by the Co-ordination European Council (CEC) to examine laboratory tests which could evaluate the quality of gasoline and diesel fuel as regards deposits concluded that no method existed which could achieve that aim. Only a Honda engine test lasting 20 hours and an analysis based on the thermogravimetry of gums in a gasoline could, in the best of cases, permit pre-selection to be carried out to develop additives.
The present invention proposes a novel laboratory test intended to evaluate the tendency of a gasoline, which may contain additives, to produce deposits on the inlet valves and in the combustion chambers.
The method of the invention can be defined in general in that a tube is placed in the injector oven of a gas chromatograph; said tube is kept at a constant temperature in a constant flow of air; the gasoline or additive-containing gasoline to be evaluated is injected through said tube; and the quantity of deposits formed is determined by weighing said tube after dismantling.